The invention relates to signal-seeking radio receivers and in particular to stepping radio receivers for the production of audio signals from radio frequencies signals. More specifically the invention relates to the use of an improved local oscillator circuit in such radio receivers.
A typical local oscillator circuit for use in a stepping radio receiver of the prior art is shown in U.S. Pat. No. 3,665,318 to Hoffman et al. The local oscillator of Hoffman works quite well with a radio receiver having 8 channels. It can also be used with stepping radio receivers having up to 10 to 12 channels. However, when the number of crystals used in the receiver substantially exceeds about 10 crystals, not only do stability problems result due to the increased capacitance of all the crystals and associated switching diodes, but additionally crystal frequencies tend to change. Therefore, when receivers were built for 16 channels using an oscillator circuit of the type shown in Hoffman et al, two separate oscillators were used, each with a capacity of 8 channels. This resulted in a more expensive product than one which would result from the use of a single oscillator, but it did provide reliable and stable operation on all of the 16 channels.
U.S. Pat. No. 3,824,475 to Pflasterer discloses a stepping radio receiver having a local oscillator which can accommodate up to 16 crystals. This basic oscillator design does not produce signals which can be used both with the low and high band VHF mixers as does the local oscillator in Hoffman, but rather it requires an additional stage to triple the output of the basic local oscillator to achieve the high band injection frequency.
The prior art literature discloses a multi-channel crystal oscillator which groups crystals into two separate groups which groups are later fed to the same oscillator element. A circuit of this type is disclosed in an article by Hilton et al. appearing in Electronic Design, Volume 13, June 21, 1969. This circuit design has, according to the disclosure, a limit of up to 5 channels per grouping. It is generally recognized in the prior art that difficulties are encountered when attempts are made to have large numbers of crystals operable with the same oscillator circuit when the crystals are switched with an electronic equivalent of a stepping switch.
U.S. Pat. No. 3,713,045 discloses a two stage oscillator circuit having a grounded collector configuration and a high impedance input for use with a tuning fork vibrator oscillator. In this circuit the base of the first transistor connects to the tuning fork, its collector to ground and its emitter to the base of the second transistor. The collector of the second transistor connects to a parallel LC circuit tuned to the resonant frequency of the tuning fork. Feed back for the oscillator is obtained from the stepped up voltage on the LC circuit coupled through a resistor back to the tuning fork. This particular configuration of two transistors apparently was found to be desirable for tuning fork oscillators. However, the crystal oscillator circuit disclosed in the same patent did not incorporate this type of circuitry.